Speed control system for induction motor

Abstract

An error amplifier is connected to both a tachometer generator for an induction motor and a command speed generator to produce an error signal corresponding to the difference between the actual motor speed and a command motor speed. A current limit generator connected to the tachometer generator generates a current limit signal which is subtracted from a motor current signal from a current sensor for the motor. A multiplier imparts to the resulting difference signal the polarity opposite to that of the error signal as determined by a polarity discriminator. The difference signal is added to the error and actual speed signals by an adder to form a command frequency signal. An inverter controls the motor in response to the command frequency signal.

What is claimed is: 
     
       1. A system for controlling a rotational speed of an induction motor, comprising an induction motor, a speed sensor connected to said induction motor to sense the actual rotational speed of said induction motor to generate an actual speed signal, a command speed generator for generating a command speed signal for the induction motor, means for producing an error signal corresponding to the difference between said actual speed signal and said command speed signal and for adding said error signal to said actual speed signal to form a command frequency signal, and an inverter responsive to said command frequency signal to deliver a voltage at a frequency corresponding to said command frequency signal to said induction motor to control the latter; wherein there are provided first means for sensing an electrical quantity relating to a torque or an output from said induction motor and for delivering a signal representing a difference between said sensed electrical quantity and a predetermined limit of the electrical quantity when said sensed electrical quantity exceeding said predetermined limit, and second means for multiplying said difference signal from said first means by a signal having the polarity determined by that of said error signal to form a product signal, and for adding said product signal to the sum of said actual and error speed signals. 
     
     
       2. A system for controlling a rotational speed of an induction motor as claimed in claim 1 wherein said first means senses a current flowing through said induction motor as said difference signal, a slip frequency control signal representing a difference between said sensed current and a predetermined current limit. 
     
     
       3. A system for controlling a rotational speed of an induction motor, comprising an induction motor, comprising an induction motor, a speed sensor disposed on said induction motor to sense the actual rotational speed of said induction motor to generate an actual speed signal, a command speed generator for generating a command speed signal for the induction motor, an error amplifier connected to both said speed sensor and said command speed generator to produce and amplify an error signal corresponding to the difference between said actual speed signal and said command speed signal, a current sensor for sensing a current flowing through said induction motor to generate a sensed current signal, a current limit generator connected to said speed sensor to generate a current limit signal, a difference amplifier connected to both said current sensor and said current limit generator to produce and amplify a slip frequency control signal in response to a difference between said sensed current signal and said current limit signal, a polarity discriminator connected to said error amplifier to discriminate the polarity of said error signal to deliver a polarity imparting signal, a multiplier connected to both said polarity discriminator and said difference amplifier to multiply said slip frequency control signal by said polarity imparting signal, adder means connected to said speed sensor, said error amplifier and said multiplier to add said actual speed signal, said error signal and an output from said multiplier together to deliver a command frequency signal, and an inverter responsive to said command frequency signal to supply to said induction motor with a predetermined voltage at a frequency determined by said command frequency signal.